I've noticed that quite a few of my photos aren't terribly well-focused. This particularly happens at long focal lengths. I'm using a Canon 500D and (mostly) a Tamron 18-270mm f/3.5-6.3 lens. I find it very hard to focus manually, mainly because I wear glasses and none of the settings on the little viewfinder-adjustment-wheel-thingy seem to be 100% right, so I use the auto-focus where I can.

5 Answers
5

Firstly I don't know about the 500D but you are probably right in that only the central point is working at f/5.6 as light is blocked to the peripheral points. You can get the camera to AF with smaller apertures (up to about f/8 if you're lucky) with the centre point if you trick the camera about what lens is being used by taping the contacts.

Can you post some images of the problem? Are the shots definitely front or back focussed instead of just soft? If so is there any consistency in the front back focussing? Sorry for the barrage of questions it's just difficult to diagnose the problem from description alone. This could indicate that the lens is out of calibration (focussing isn't a feedback loop, the AF takes a measurement and then tells the lens to go there.

Have you thought about manual focus with live view? It may be tricky at the long end due to camera movement (a monopod/tripod will help here) but you should be able to view the screen from a comfortable distance.

At the end of the day, any lens with such a large zoom range will never perform as well as a dedicated telephoto so you will undoubtably benefit from upgrading to something faster.

from your question it's hard to know whether you're just not focusing right, or whether you just feel your pictures are quite sharp enough.

If you're just not focusing right then I can't help you..

HOWEVER, if you're talking sharpness I've got plenty of advice!

Sharpness

What I'm assuming might be happening with you is that you're finding your pictures are perhaps lacking sharpness.

Gear

I've been quite interested in this subject myself, and asked a few question here recently about lens sharpness which I think might explain some stuff, check them out, you might not relate to the questions, but the answers are excellent:

Subject Matter

I used to alway shoot urban landscape stuff with my aperture wide open, simply because I needed the light. The pictures always felt a bit "soft". I've come to understand that sometimes you just need to close that aperture!

Yes, you may need a tripod, or a higher ISO in your case, or in my case a faster film, but especially when your focus is almost at infinity, and you're doing urbanscape/landscape consider upping your f-stop. If you've read my question on the "sweet spot" of a lens, you'll notice the sharpest option seems to be around the upper half of your lens f range.

Generally experiment with your lens around f-11, f-16, see what you think of the results.

Lighting Conditions

Consider the lighting conditions you're shooting in. Usually, if the light is harsh and/or flat, like a sunny day around noon... it's gonna be hard to "perceive" sharpness in your image.

Indeed, sharpness can be a relative thing. You'll notice that during the "golden hour" everything seems to have a sharper edge. This is because this softer, more horizontal light, is giving everything soft shadows, and is actually highlighting the "depth" of subjects. It's like how painters use shadows to give the illusion of 3D. This is true with photography too. It doesn't have to just be the golden hour though, notice night shots look more sharp too because of the quality of the light.

this question might help you too, it helped me! generally, trying experimenting with different light conditions, you'll find some will result in images which feel sharper.

You say that the images are soft, not that the AF mechanism is hunting. If the AF can lock, then there's no reason for it to do so inaccurately in a way which would vary with focal length or aperture. It's either able to focus, and correctly calibrated, or it is not. Assuming your description is precise, then that suggests the problem is with your lens or your technique.

gear
I have no experience of that gear, but it's extremely hard to get high quality across such a large zoom range. I'd suspect that the lens is soft at full-bore. You can easily test this. Stick a target of some description on a wall and shoot it with the camera set on a tripod. Vary separately aperture and zoom and look at what you get. You should find a sweet spot for aperture, and you'll probably find that sharpness varies with focal length too. I had an L-series Canon zoom lens which when tested was so soft at full-bore as to be unusable (something I'd obviously noted in use, but it's nice to have the evidence for the lawyers).

technique At 270mm on an APS-C camera you'd be needing a shutter speed of about... 1/(270*1.6) or more than 1/400th of a second. What are you using? If you're trying to hand hold it at much less than that then you're not likely to get particularly sharp shots.

If this is the case with your lens, and if you're keeping it reasonably static, you can reassure yourself of your focus by zooming in to a target point with clear contrast, focussing, and re-adjusting the zoom to where you want it.

This is a common technique when working with 35mm movie cameras (where the camera is often statically mounted), though I don't know whether stills photographers use it much.